1. Field of the Invention
The present invention generally relates to bicycles. More specifically, the present invention relates to a bicycle frame including an improved support for a down tube of the bicycle frame and a method for constructing such a bicycle frame.
2. Description of the Related Art
One type of bicycle frame is an assembly of multiple components assembled together into a desired shape. The individual components of such a bicycle frame may comprise metal tubes that are cut, shaped and joined together, typically by a welding process. In general, a bicycle frame is configured to support a seat assembly, a pedal crank assembly, a rear wheel and a steering assembly. The steering assembly typically includes a handlebar, front fork and front wheel coupled to one another and permitted to rotate relative to the bicycle frame. If the associated bicycle is intended for off road use, the bicycle frame may provide rear wheel suspension. Such a frame may include a main frame and sub-frame pivotally connected to the main frame. The sub-frame supports the rear wheel and a shock absorber may be connected between the main frame and the sub-frame to influence relative movement between the main frame and the sub-frame.
A central, lower portion of the bicycle frame typically defines a laterally extending opening configured to support the pedal crank assembly. Typically, the opening is defined by a tube referred to as a bottom bracket shell. The opening of the bottom bracket shell is configured to receive and support a bottom bracket assembly, which includes one or more bearings to rotatably support the pedal crank assembly.
In addition to supporting the pedal crank assembly, the bottom bracket shell is often used to connect a down tube of the main frame to a seat tube of the main frame. Typically, the ends of the down tube and seat tube are shaped, or mitered, to mirror an external surface of the bottom bracket shell. The mitered ends of the down tube and seat tube are butted against the external surface of the bottom bracket shell and welded in place. This type of connection between the tubes and the bottom bracket shell requires precision fixturing of the down tube and seat tube relative to the bottom bracket shell to ensure proper alignment of the frame, once the tubes are welded to the bottom bracket shell. The seat tube and down tube must be precisely positioned along the bottom bracket shell in a lateral direction and must be positioned radially along the bottom bracket shell at a desired angle from one another. Furthermore, if the bottom bracket shell is a cylindrical tube, as is common, the surface area available to receive and support the seat tube and down tube is limited. As a result, the diameters of the seat tube and down tube may be limited by the size of the bottom bracket shell, which generally is sized to receive an industry-standard bottom bracket assembly. In addition, the simple butted joint between the down tube and the bottom bracket shell often results in a welded connection that exhibits undesirable strength characteristics.